In an internal combustion engine installed in a vehicle or the like, evaporated fuel leaks into the atmosphere from a fuel tank, and a float chamber of a carburetor contains a large amount of hydrocarbons (HC) and is one of the causes of air pollution and also results in loss of fuel. Therefore, various techniques to prevent such problem are known. As a typical technique, there is an evaporating system (also referred to as an "evaporation fuel control apparatus") in which evaporated fuel in a fuel tank is adsorbed and held in a canister containing an adsorbent such as activated carbon or the like, and the evaporation fuel adsorbed and held in the canister is purged at the time of operation of the internal combustion engine and is supplied into the internal combustion engine.
The known evaporating system has: a canister which communicates with a fuel tank of the internal combustion engine by an introducing passage and communicates with an intake passage by a discharge passage; and a purge control valve is provided in the discharge passage. In the evaporating system, the purge control valve is opened or closed in accordance with the operating state of the engine and the evaporated fuel in the canister is purged into the intake passage and is combusted.
In the internal combustion engine having the above evaporating system, there is provided an air-fuel ratio control apparatus. The air-fuel ratio control apparatus controls an air-fuel ratio feedback value on the basis of a detection signal of an exhaust sensor provided in the exhaust passage, and a feedback control is executed so as to set an air-fuel ratio to a target value by making, for example, a fuel injection valve operative on the basis of the air-fuel feedback value.
As an air-fuel ratio control apparatus of an internal combustion engine, there is an apparatus disclosed in document JP-A-4-153554. A failure diagnosis apparatus of an evaporation purging system disclosed in the above document comprises: a control valve for diagnosis which is provided in an atmosphere release passage of a canister; pressure detecting means for detecting a pressure in the atmosphere release passage between the canister and the control valve; valve control means for respectively opening or closing the control valve for purge and for diagnosis at the time of diagnosis; and failure detecting means for detecting a failure of the system by comparing a pressure value detected by the pressure detecting means and a reference value upon diagnosis; and wherein the control valve for diagnosis is closed and a failure is detected when the control valve for purge is opened and when it is closed, respectively.
Further, an air-fuel ratio control apparatus is also disclosed in document JP-A-4-279755. According to this document, the apparatus has a fuel evaporation discharge suppressing apparatus and properly controls an air-fuel ratio of the mixture gas when shifting from a purge gas supply state to a shut-off state, thereby improving drivability of the engine and exhaust gas characteristics.
Further, there is further disclosed an air-fuel ratio control apparatus in document JP-A-4-362264. According to a failure diagnosis apparatus of an evaporation purging system disclosed in the above document, just after the start of the engine and when engine temperature is equal to or less than a predetermined value, a control valve for diagnosis is closed, a control valve for purge is opened, a negative pressure of an intake pipe is introduced to a vapor passage, and thereafter the control valve for purge is opened and this state is held for a predetermined time, thereby detecting a failure by a change in pressure value within the predetermined time.
In the conventional evaporating system for controlling the air-fuel ratio of an internal combustion engine, there is a system which judges leakage. That is, as leakage judging conditions, as shown in FIG. 10(c), an atmosphere opening/closing valve is set into a closing state and a pressure in the fuel tank is set to a negative pressure state up to a specified value (refer to FIG. 10(a)). As shown in FIG. 10(b), a purge duty is set to 0%, thereby detecting a pressure change in a predetermined time. In this instance, as shown by a broken line in FIG. 10(a), when the pressure change is large (.DELTA.P.sub.yes), it is determined that leakage is occurring in the evaporating system. On the contrary, when there is no pressure change (.DELTA.P.sub.no), it is determined that no leakage is occurring in the evaporating system.
In the judgment of the leakage, when the leakage judging conditions are satisfied, an atmosphere opening/closing valve provided in an atmosphere passage of the canister is closed, a purge control valve provided in a discharge passage is opened, and an intake negative pressure of an intake passage is introduced to the fuel tank side.
Subsequently, after the pressure in the fuel tank was set into a negative pressure state of a specified value by the intake negative pressure which is introduced, the purge control valve is closed, the fuel tank side of the purge control valve is held in a negative pressure state, a pressure .DELTA.P in the fuel tank within a predetermined time is detected by a pressure sensor, and a leakage is discriminated by a change state of the pressure .DELTA.P which is detected. As for the judgment, when a change in pressure .DELTA.P is large, it is determined that there is leakage. When the change in pressure .DELTA.P is small, it is decided that there is no leakage.
When a leakage occurs in the evaporating system, as shown in FIGS. 10(d) and 10(e), an air-fuel ratio feedback and an air-fuel ratio are large and suddenly change.
Therefore, when the external temperature (or fuel tank temperature) is high, a large quantity of evaporation occurs and, as shown in FIG. 10(d), the air-fuel ratio feedback is largely corrected to the negative side.
When the leakage judgment is executed in such a state, the evaporation is shut off by the closing operation of the atmosphere opening/closing valve. As shown in FIG. 10(e), the air-fuel ratio is largely deviated to the lean side.
At this time, there are inconveniences such that in the ordinary feedback control, trackability is bad, drivability is defective by making the air-fuel ratio lean, the exhaust gas purifying function deteriorates, and this is practically disadvantageous.
To eliminate the above inconveniences, the present invention provides an air-fuel ratio control apparatus of an internal combustion engine in which a canister to adsorb and hold evaporated fuel is provided in a passage which communicates an intake passage of the internal combustion engine and a fuel tank, an atmosphere opening/closing valve which operates so as to open said canister to the atmosphere is provided, a purge valve is provided in a passage between the intake passage and the canister, a pressure sensor communicates with the passage between the canister and the fuel tank, and when leakage is judged, the atmosphere opening/closing valve is closed and a pressure in the fuel tank is reduced to a predetermined negative pressure value, and thereafter the purge valve is closed and a leakage state of an evaporating system is discriminated, the improvement comprising a fuel supply section to supply fuel to the internal combustion engine is provided, and there is also provided a control section for controlling in a manner such that when leakage judging conditions are satisfied, a preset promising correction amount is added to a fuel supply amount from the fuel supply section synchronously with the closing operation of the atmosphere opening/closing valve, thereby reducing fluctuation of the air-fuel ratio when leakage is judged.
The invention also provides an air-fuel ratio control apparatus, as aforesaid, wherein the promising correction amount which has been preset by an atmospheric temperature is added to a fuel supply amount from the fuel supply section synchronously with the closing operation of said atmosphere opening/closing valve, and after the promising correction amount is added to the fuel supply amount and controlled, the fuel supply amount is gradually attenuated from the value added with the promising correction amount until the total fuel supply amount is less than a predetermined supply amount, thereby reducing fluctuation of the air-fuel ratio by the leakage judging operation.
The invention further provides an air-fuel ratio control apparatus, as aforesaid, wherein there is provided a control section for controlling in a manner such that a feedback correction amount before the leakage judgment is started is stored and, at the end of said leakage judgment, the correction amount is returned to the stored feedback correction amount before the start of the leakage judgment.
Further, the invention provides an air-fuel ratio control apparatus, as aforesaid, wherein there is provided a control section for controlling in a manner such that when leakage judging conditions are satisfied, a mean value of a feedback correction amount in a predetermined time before the closing operation of the atmosphere opening/closing valve is calculated and stored and, at the end of the leakage judgment, the feedback correction amount is forcedly set to said mean value.
The invention further provides an air-fuel ratio control apparatus, as aforesaid, wherein there is provided control means for controlling in a manner such that an air-fuel ratio feedback value is calculated on the basis of a detection signal of an exhaust sensor provided in the exhaust passage of the internal combustion engine, a feedback control is performed so as to set an air-fuel ratio to a target value, and simultaneously with the closing operation of the atmosphere opening/closing valve when the leakage judgment is executed by said evaporation fuel control apparatus, a feedback control is performed so that the air-fuel ratio is set to a target value by returning said air-fuel ratio feedback value to a reference value.
By the invention as mentioned above, when the leakage judging conditions are satisfied, the preset promising correction amount is added to the fuel supply amount from the fuel supply section synchronously with the closing operation of the atmosphere opening/closing valve by the control section, thereby controlling so as to reduce the fluctuation of the air-fuel ratio upon judgment of leakage. Deterioration of drivability is prevented, and deterioration of the exhaust gas purifying function is prevented.
The control section controls in a manner such that when the leakage judging conditions are satisfied, the promising correction amount which is preset by the atmospheric temperature is added to the fuel supply amount from the fuel supply section synchronously with the closing operation of the atmosphere opening/closing valve, and after the promising correction amount is added to the fuel supply amount and controlled, the fuel supply amount is gradually attenuated from the value to which the promising correction amount was added until the total fuel supply amount is less than a predetermined supply amount, and fluctuation of the air-fuel ratio by the leakage judging operation is reduced, thereby preventing deterioration of the drivability and the exhaust gas purifying function.
Further, the control section stores the feedback correction amount before the start of the leakage judgment and, upon completion of the leakage judgment, the correction amount is returned to the feedback correction amount before the start of the leakage judgment.
Further, the control section controls in a manner such that the mean value of the feedback correction amount within a predetermined time before the closing operation of the atmosphere opening/closing valve is calculated and stored when the leakage judging conditions are satisfied, and upon completion of the leakage judgment, the feedback correction amount is forcedly set to the mean value.
In the air-fuel ratio control apparatus of an internal combustion engine having the evaporation fuel control apparatus, a feedback control is executed by the control means so that the air-fuel ratio is set to a target value by calculating the air-fuel ratio feedback value on the basis of the detection signal of the exhaust sensor. A feedback control is executed by the evaporation fuel control apparatus so that the air-fuel ratio is set to a target value by returning the air-fuel ratio feedback value to the reference value simultaneously with the closing operation of the atmosphere opening/closing valve when the leakage judgment is performed. Due to this, in order to perform the leakage judgment, the atmosphere opening/closing valve is closed, the feedback control can be executed while promptly tracing the air-fuel ratio which was largely deviated to the lean side, and the fluctuation of the air-fuel ratio can be reduced.
An embodiment of the invention will now be described in detail hereinbelow with reference to the drawings.